[0001] The invention relates to a longitudinal member for use in spinal or trauma surgery
and a stabilization device with such a longitudinal member.
[0002] A dynamic stabilization system for segments of the spinal column which comprises
a flexible rod made of an elastic material and bone anchors to anchor the rod in the
vertebrae is known from
EP 1 364 622 A2 and
EP 1 527 742 A1, respectively. The material of the rod is a biocompatible polymer material, for example
a material on the basis of polyurethane. The rod has a corrugated surface with the
corrugations extending in a direction transverse to the rod axis.
[0003] Usually, the elastic rods are manufactured by injection molding whereby the molten
plastic material is injected at high pressure into a mold which is the inverse of
the desired shape. As shown in Fig. 1, after injection molding, the polymer chains
100 of the material are entangled and may include filling particles 101 and transverse
links 102 between them. A rod 103 which is made by injection molding comprises an
isotropic structure of the polymer chains and is therefore non homogeneous in a sense
that it comprises defects in its macromolecular structure. The known elastomer rods
exhibit a local flow of material when pressure is exerted onto their surface in the
process of fixing the rod within a receiving part of a bone anchoring element. This
local flow of material may cause a loosening of the fixation of the rod within the
bone anchoring element.
[0004] It is the object of the invention to provide a longitudinal member for use in spinal
or trauma surgery and a stabilization device using such a longitudinal member manufacturing
which has improved mechanical properties as well as reduced manufacturing costs compared
to the known polymer rods.
[0005] The object is solved by a longitudinal member according to claim 1, 3 or 11 and a
stabilization device according to claim 14. Further developments are given in the
dependent claims.
[0006] The longitudinal member has the advantage that its tendency to flow when being fixed
to the bone anchor is reduced in comparison to the known injection molded elastomer
rods. In addition, the longitudinal member in form of the extruded elastomer rod exhibits
a lower permanent set, which characterizes the deformation remaining after removal
of the deforming stress, and a higher stiffness characterized by the e-modulus compared
to the injection molded rod at identical dimensions of the rod. Therefore, under identical
load conditions, an extruded elastomer rod with smaller dimensions can be used. Furthermore,
the strength against mechanical tensile and/or compressive loads and the abrasion
resistance is enhanced. The costs for manufacturing are reduced with regard to the
necessary tools and machines which are less expensive compared to the costs for the
manufacturing by injection molding.
[0007] The rod can be cut to the desired length before or during surgery.
[0008] Further features and advantages of the invention will become apparent and will be
best understood by reference to the following detailed description of embodiments
taken in conjunction with the accompanying drawings.
Fig. 1 shows a schematic representation of an arrangement of polymer chains of a polymer
plastic material after injection molding.
Fig. 2 shows a schematic cross-section of a spinal rod made of a polymer plastic material
produced by injection molding.
Fig. 3 shows a perspective view of a rod according to the present invention.
Fig. 4 shows a schematic cross-sectional view of the rod according to Fig. 3 in a
plane including the longitudinal axis of the rod.
Fig. 5 shows a schematic cross-sectional view of the rod according to Fig. 3 in a
plane perpendicular to the longitudinal axis.
Figs. 6a) to g) show examples of cross-sections of the rod.
Fig. 7 shows a stabilization device for the spinal column including a rod according
to the invention and two monoaxial bone screws.
Fig. 8a shows a stabilization device for the spinal column including a rod according
to the invention and two polyaxial bone screws.
Fig. 8b schematically shows the forces acting onto the rod under axial load and flexion.
Fig. 9 shows application of the stabilization device according to the invention to
the spinal column for the purpose of correction of scoliosis, wherein the rod according
to the invention is in a first, pre-stressed condition.
Fig. 10 shows the stabilization device of Fig. 9 in a second condition.
Fig. 11 is a schematic view showing a bone screw and a clamp anchored in a vertebra
and fixing the rod.
Fig. 12 shows a modified example of a bone anchoring element receiving the rod.
[0009] Fig. 3 to 5 show an embodiment of the invention used as a spinal rod 1. The rod has
a substantially circular cross section and a length which is suitable to span a distance
between at least two vertebrae. The diameter of the rod can be selected so as to be
compatible with that of known metallic spinal rods. In this case, the rod 1 can be
connected to known bone screws. In the embodiment shown the cross-section of the rod
is constant over the length of the rod.
[0010] The rod is made of a biocompatible plastic material which can be molded by extrusion.
For example, the material can be a thermoplastic material such as polyaryletheretherketone
(PEEK) Preferably, the material is flexible, such as an elastomer. Suitable elastomers
are for example polymer materials on the basis of polyurethane, polycarbonate-urethane
(PCU) or silicone. The rod exhibits a three-dimensional elasticity in such a way that
a restoring force acts when the rod is put under load which restores the original
shape of the rod.
[0011] As can be seen in particular in Fig. 3 and 4, the macromolecular construction of
the rod 1 is characterized by polymer chains 2 of the elastomer material which are
substantially aligned in the longitudinal direction of the rod 1. The macromolecular
structure of the rod is therefore substantially uniform in the longitudinal direction.
The polymer chains 2 form a fiber-like structure with the fibers oriented in the longitudinal
direction, thus being load-oriented.
[0012] The rod 1 is preferably manufactured by extrusion. In the well known manufacturing
process of extrusion the solid or fluid raw material is filled in an extruder and
then pressed through an opening. The parameters such as temperature and pressure during
the extrusion process depend on the material used and will be recognized by those
skilled in the art.
[0013] Hence, the rod 1 can be distinguished from a conventional rod made of the same material
but manufactured for example by injection molding, as shown in Fig. 2. The extruded
rod has enhanced mechanical strength compared to a rod made of the same material by
means of injection molding. For example, the strength against mechanical tensile and/or
compressive loads is enhanced. Furthermore, the wear resistance is enhanced. Therefore,
the rod implant has an improved lifetime.
[0014] The rod can have other shapes than a circular cross section. As can be seen in Figs.
6a) to g), different cross sections such as circular (Fig. 6a)), square (Fig. 6b)),
rounded square (Fig. 6c)), oval-shaped (Fig. 6d)), rectangular (Fig. 6e)), rounded
rectangular (Fig. 6f)) or star-shaped (Fig. 6g)) or triangular are possible. Preferably
the cross-section is constant over the length of the rod. With a non-circular cross-section
of the rod a rotation of the rod in the bone anchoring element to which it is connected
can be prevented. In addition, the shape of the cross-section can be used to achieve
bending properties in flexion/extension movement and lateral bending which can differ
from each other.
[0015] A stabilization device using the rod according to the invention comprises at least
two bone anchoring elements for connection of the rod to the bone. As can be seen
in Figs. 7 and 8, according to a first example the bone anchoring elements are monoaxial
bone screws 10, 10' each comprising a threaded shaft 11 which is to be anchored in
a vertebra and a receiving part 12 which is rigidly connected to the threaded shaft.
The receiving part 12 has a substantially U-shaped recess to receive the rod 1. A
locking element, for example an inner screw to be screwed into the recess or, as shown,
an outer nut 13 is provided to fix the rod 1 in the recess. The bone anchoring elements
are made of a biocompatible rigid material, for example of a biocompatible metal,
such as titanium or a metal alloy.
[0016] In use, first, the bone anchoring elements 10, 10' are screwed into the vertebrae
which shall be stabilized. Then the rod 1 is inserted into the receiving parts 12
and, after adjustment of its position, fixed in the receiving part by means of the
locking element 13. Due to the uniformly aligned macromolecular structure of the rod
the tendency to flow under pressure of the locking element is reduced. Therefore,
the risk of loosening of the fixation between the rod and the bone anchoring element
is reduced. Since the rod exhibits elasticity under flexion, extension and torsion
of the spinal segment, the spinal segment can be dynamically stabilized. The elasticity
required for a certain application can be obtained by selecting the material and/or
the size and/or the shape of the cross-section of the extruded rod.
[0017] In the stabilization device of Fig. 7 the rod 1 is used in a straight state. The
vertebral segment can perform a limited motion in all planes controlled by the elasticity
of the rod.
[0018] Fig. 8a shows a second example of a stabilization device using the extruded rod 1.
The stabilization device has at least two polyaxial bone anchoring elements 14 and
14' having a threaded shaft 15 to be anchored in the bone and a spherically-shaped
head 16 at one end. The head 16 is pivotably held in a receiving part 17 which also
receives the rod 1 in a recess. Preferably, a pressure element (not shown) is provided
which presses onto the head to fix the head in the receiving part in its angular position.
A locking element (not shown) is also provided to fix the rod in the recess.
[0019] In use, like in the first example, the bone anchoring elements 14 and 14' are screwed
into the vertebrae and thereafter the rod 1 is inserted. Since the head 16 is pivotably
held in the receiving part 17 the position of the receiving parts can be adjusted
relative to the heads. After adjustment of the position of the receiving parts relative
to the heads and of the rod relative to the receiving part, the connection is locked
by means of the locking element.
[0020] Fig. 8b schematically shows the forces acting onto the rod under axial (A) and flexural
(F) load during motion of the spinal segment shown in Fig. 8a. As can be seen, the
force components of the axial and flexural load are mainly oriented in the direction
of the alignment of the polymer chains 2. This renders the extruded rod particularly
suitable for the application in dynamic stabilization of the spinal column. This also
applies to the stabilization device shown in Fig. 7 using monoaxial screws.
[0021] Fig. 9 and 10 show an example of a clinical application of a correction device. The
correction device which includes two bone anchoring elements 10 and the extruded rod
1 is applied to a spinal section exhibiting scoliosis. The elastic rod is bent out
of its neutral straight shape so as to be adapted to the curvature of the spinal deformity
as shown in Fig. 9. By narrowing the distance between the screw heads of the correction
device as indicated by the arrows in Fig. 9, a pretension is generated in the rod
which urges the deformed part of the spine into a straight position as shown in Fig.
10. For the bone anchoring elements monoaxial or polyaxial screws can be used. Polyaxial
screws have the advantage that the shaft and the head can be aligned for receiving
the rod.
[0022] Fig. 11 shows a schematic view in the direction of the longitudinal axis of the spine
of one bone anchoring element anchored in a vertebra. The extruded rod is clamped
in the receiving part 12 by the locking element 13. The polymer chains 2 are substantially
aligned in the longitudinal direction of the rod. If required, additional fixation
in the bone can be provided by means of clamps 20.
[0023] Fig. 12 shows a modified example of a bone anchoring element with the rod inserted
into the recess of the receiving part. The receiving part 18 comprises a recess 19
with a cross-section which differs from the cross-section of the rod. In the example
shown the cross section of the recess is oval-shaped while the cross-section of the
rod is circular with a diameter smaller that that of the recess. Fixation can be achieved
via a locking element (not shown) either directly or with a filling piece (not shown)
between the locking element and the rod.
[0024] The invention is not limited to the above described embodiments and examples of application.
The features of the examples described can be combined with each other. Although the
rod is shown to connect two bone anchoring elements, it can have a length sufficient
to connect more than two bone anchoring elements. Since the rod is made of an elastomer,
the length can be adapted before or at the time of surgery by cutting the rod.
[0025] The rod can be made fully or partially of the polymer material. For certain applications
it is sufficient that a section of the whole rod is made of extruded polymer material.
The length of the section depends on the specific application and the required flexibility.
A non isotropic shape for the cross-section, such as for example a rectangular shape,
can be used for providing a rod with elastic characteristics which differ dependent
on the direction.
[0026] The rod can be hollow and can include a core in its hollow interior for obtaining
further characteristics.
[0027] With the manufacturing method of extrusion it is possible to produce rods with different
shapes and diameters of the cross-section at low costs, since it is not necessary
to use complex molds and expensive machines like in the injection molding process.
[0028] For the bone anchoring elements all known types can be used which are typically used
with the known metallic rods.
[0029] The invention is also not limited to the application for the spine. The rod can also
be used in stabilizing a fractured bone, for example instead of a metallic rod in
a fixateur externe or interne.
[0030] The term polymer material as described above means a single polymer material or mixtures
of polymer materials including co-polymers and so-called block co-polymers having
hard and soft segments. It also includes polymer materials with additions such as
filling particles or strengthening fibres like carbon fibres or the like. Strengthening
fibers can be used to enhance the stiffness, if required.
1. A longitudinal member (1) for use in spinal or trauma surgery which is sized to span
a distance between at least two vertebrae or two bone parts,
characterized in that the longitudinal member is made at least partially of an extruded polymer material.
2. The longitudinal member of claim 1, wherein the polymer chains (2) of the material
are substantially aligned in the longitudinal direction of the member (1).
3. A longitudinal member (1) for use in spinal or trauma surgery which is sized to span
a distance between at least two vertebrae or two bone parts,
characterized in that the longitudinal member is made at least partially of polymer material the polymer
chains (2) of which are substantially aligned in a longitudinal direction of the member
and wherein the longitudinal member (1) is obtainable by extrusion.
4. The longitudinal member of one of claims 1 to 3, wherein the polymer material is an
elastomer.
5. The longitudinal member of one of claims 1 to 4, wherein said material is a polymer
on the basis of polyurethane, polycarbonate-urethane (PCU) or silicone.
6. The longitudinal member of one of claims 1 to 5, wherein the longitudinal member (1)
is a rod.
7. The longitudinal member of one of claims 1 to 6, wherein the cross-section of the
member is constant over the length of the member.
8. The longitudinal member of one of claims 1 to 7, wherein the member exhibits bending
elasticity.
9. The longitudinal member of one of claims 1 to 8, wherein the cross-section is substantially
circular.
10. The longitudinal member of one of claims 1 to 9, wherein the whole length of the member
is made of said polymer material.
11. A longitudinal member (1) for use in spinal or trauma surgery which is sized to span
a distance between at least two vertebrae or two bone parts, the longitudinal member
exhibiting bending and axial elasticity,
characterized in that the longitudinal member is made of load-oriented fibers and uniform cross-section.
12. The longitudinal member (1) of claim 11, characterized in that it is obtained by extrusion.
13. The longitudinal member of claim 11 or 12, wherein the member is made of a polymer,
preferable of an elastomer.
14. Stabilization device for stabilizing vertebrae or bone parts, comprising
at least two bone anchoring elements having a shaft for anchoring in the bone and
a receiving part for connection with a longitudinal member, characterized in that a longitudinal member according to one of claims 1 to 13 is provided for connection
with the bone anchoring elements.
15. The stabilization device of claim 14, wherein the receiving part (12; 17; 18) comprises
a recess (19) for receiving the longitudinal member, the cross-section of the part
of the recess (19) receiving the longitudinal member being different from the cross
section of the longitudinal member.
16. Method of manufacturing a longitudinal member according to one of claims 1 to 13,
characterized by the steps providing a biocompatible polymer material and
extruding it into the shape of the longitudinal member.
17. The method of claim 16, wherein an elastomer material, such as a polymer on the basis
of polyurethane, polycarbonate-urethane (PCU) or silicone is used.